Just jumping on the bandwagon here. Would love to have the ability to password protect at a notebook level. Then have password protected notebooks be excluded from search unless you enter the password to unlock those notebooks. And heck, maybe even also have protected notebook results, even when unlocked, be available (optionally) after the click of a button or link. For example, let's say you have Notebook A, Notebook B, and Notebook C. B and C are protected with different passwords. Notebook A is unprotected. You run a search. Results look like this:
Result 1, result 2, result 3, etc. (all from Notebook A).
[see results from locked Notebook B] -> click on this and you'll be prompted for a password to unlock notebook B.
[see results from locked Notebook C] -> click on this and you'll be prompted for a password to unlock notebook C.
In the left column, in the notebooks view, you would see a little lock symbol (like a padlock) next to locked notebooks. When unlocked, the lock symbol padlock would look unlocked. Click on the padlock again and you lock that notebook.
On mobile, the experience might be a little different. You might allow the use of a PIN instead of a password, or Touch ID (iOS), face unlock, pattern unlock, etc.